Inconsistencies in air quality metrics: ‘Blue Sky’ days and PM10 concentrations in Beijing

International attention is focused on Beijing’s efforts to improve air quality. The number of days reported as attaining the daily Chinese National Ambient Air Quality Standard for cities, called ‘Blue Sky’ days, has increased yearly from 100 in 1998 to 246 in 2007. However, analysis of publicly reported daily air pollution index (API) values for fine particulate matter (diameter≤10 µm, PM10), indicates a discrepancy between the reported ‘Blue Sky’ days (defined as API≤100, PM10≤150 µg m−3) and published monitoring station data. Here I show that reported improvements in air quality for 2006–2007 over 2002 levels can be attributed to (a) a shift in reported daily PM10 concentrations from just above to just below the national standard, and (b) a shift of monitoring stations in 2006 to less polluted areas. I found that calculating daily Beijing API for 2006 and 2007 using data from the original monitoring stations eliminates a bias in reported PM10 concentrations near the ‘Blue Sky’ boundary, and results in a number of ‘Blue Sky’ days and annual PM10 concentration near 2002 levels in 2006 and 2007 (203 days and ∼167 µg m−3 calculated for 2006—38 days fewer and a PM10 concentration ∼6 µg m−3 higher than reported; 191 ‘Blue Sky’ days and ∼161 µg m−3 calculated for 2007—55 days fewer and a PM10 concentration ∼12 µg m−3 higher than reported; 203 days and 166 µg m−3 were reported in 2002). Furthermore, although different pollutants were monitored before daily reporting began and less stringent standards were implemented in June 2000, reported annual average concentrations of particulate (diameter≤100 µm, TSP) and nitrogen dioxide (NO2) indicate no improvement between 1998 and 2002. This analysis highlights the sensitivity of monitoring data in the evaluation of air quality trends, and the potential for the misinterpretation or manipulation of these trends on the basis of inconsistent metrics.


Introduction
In 1998, Beijing launched a 'Defending the Blue Sky' campaign with air quality data becoming openly available on a weekly basis in February 1998 [1][2][3], and on a daily basis in June 2000 [4]. In January 2003, the data from individual monitoring stations also became publicly 1 Present address: University of California, Los Angeles-School of Law, Los Angeles, CA 90095, USA.
In 1998, Beijing was ranked as having the 3rd worst air quality in a global ranking of 157 cities across 45 countries [3]. The annual average particulate concentration (diameter 100 µm, TSP) that year was 35% higher than Mexico City, which was ranked as having the worst air quality in the world [3,9]. It has been widely reported that the air and distant sources [7,26] impacts Beijing air quality, and has complicated control efforts taken by the Beijing government. These issues have affected the city air quality to varying degrees throughout the 1998-2007 periods, and are not addressed in the present study which focuses on reported air quality trends and Beijing's efforts to meet the Chinese air quality standards. Severe dust storms during the spring of 2006 are likely partially responsible for high average particulate levels for that year, but these days of highly elevated particulate concentrations should not affect days with particulate concentrations near the PM 10 = 150 µg m −3 national ambient air quality standard.
According to the national Chinese State Environmental Protection Administration's 2005 Automated Methods for Ambient Air Quality Monitoring (HJ/T 193-2005), which went into effect on January 1, 2006, cities with a population of over 3 million people are required to use at least eight monitoring stations to measure urban air quality [33]. In addition, new specifications were added regarding the minimum distance from roadways that air pollution should be monitored. For roadways with an average of 3000 vehicles per day, monitoring stations should be a minimum of 25 m from the road; for roadways with an average of 15 000 vehicles per day a minimum of 80 m; and for roadways with an average of 40 000 vehicles per day a minimum of 150 m [33]. In 2005, Beijing municipality had 15.4 million permanent residents and the 293 primary roads in urban districts of the city had a total length of 596 km that carried on average 5422 vehicles per hour [23b], a number that would increase 11.4% to 6040 vehicles per hour in 2006 [23a].
The reported Beijing air quality is an average of data from selected monitoring stations [27]. From 1984 to 2005, the 7 stations used to measure air quality remained constant. These stations monitored areas with different characteristics, e.g., traffic, residential, commercial, and industrial [1,[28][29][30][31]. Although the number of monitoring stations increased from 8 in 1984, one of the original monitoring stations was a background station located near the Ming tombs 80 km outside of the city, to 27 [28,35]. The monitoring stations used for determining daily city pollution levels and 'Blue Sky' days are also used to calculate annual average pollution concentrations [1,26]. In 1998, without the two monitoring stations in transportation areas, the annual average particulate (TSP) concentration for the city would have been 7% lower, the annual average NO x concentration would have been 24% lower, and the annual average SO 2 concentration would have been 10% lower than reported for that year.
Reports have raised questions regarding the accuracy of scientific and air quality reporting in China [36][37][38][39]54]. However, the annual number of 'Blue Sky' days, along with annual pollutant concentrations, continue to be used in China to evaluate air quality trends [3,5,14,18,22], model air pollution [31,40], calculate the health and economic impacts of air pollution [2, [41][42][43], and establish air quality control plans [44]. No known study has analyzed the sensitivity of Beijing's air quality monitoring data to the analysis of air quality trends, which I examine by calculating the impact of the change in monitoring station locations on reported air quality, or examined the air pollution index reporting system for other irregularities, including the revision of standards in June of 2000 [45,46]. The relative importance of nitrogen oxides (NO 2 /NO x ) and sulfur dioxide (SO 2 ) in public air quality reporting will also be addressed, along with a discussion of monitoring station locations.

Data
This study used daily and weekly air quality data, reported as Air Pollution Index values, publicly available from the State Environmental Protection Agency (SEPA, www.zhb.gov.cn) and Beijing Environmental Protection Bureau (BJEPB, www. bjepb.gov.cn). Chinese API values are a scientific measure of air quality designed to inform the public about air pollution and the potential impacts on human health [27]. The conversion from API values to pollutant concentrations is detailed in SEPA technical regulations in both Chinese and English, and has been used and described in several scientific studies [46][47][48]. The Chinese API is based on the air quality index (AQI) used in the United States, and although the standards vary, the calculation methodology is the same [49]. Similar index systems are also used in other countries [46].
In major cities in China, concentrations of the pollutants PM 10 (TSP from 1998 to 2000), NO 2 (NO x from 1998 to 2000), and SO 2 are monitored and converted to an air pollution index (API) value between 1 and 500 (table 1) [27]. From 1998 to 2000, ozone (O 3 ) and carbon monoxide (CO) were also used in API reporting [38]. Each day (week from 1998 to 2000), the highest API value is reported, and the primary pollutant is identified if its API is >50, indicating potential risk to human health.
where API = air pollution index, C p = the concentration of pollutant p, I Hi = API value corresponding to B P Hi , I Lo = API value corresponding to B P Lo , B P Hi = the breakpoint that is greater than or equal to C p , B P Lo = the breakpoint that is less than or equal to C p . An API value less than or equal to 100 indicates attainment of the national air quality standard-a 'Blue Sky' day. For PM 10 [21].
The table of pollutant concentrations and equivalent API breakpoints is the same in the Chinese and English versions of the SEPA technical regulations; however, the sample calculation in the English version incorrectly uses a PM 10 concentration of 250 µg m −3 for the API breakpoint of 200. The correct PM 10 = 350 µg m −3 for the API breakpoint of 200 is used in the Chinese version, has been applied in scientific studies [46][47][48], and is consistent with the US EPA methodologies. However, several studies have included the incorrect breakpoint [6,41].

Analysis
I examine the frequency distribution of API values focusing on values near the 'Blue Sky' boundary, and calculate the daily Beijing API on days when the primary pollutant was PM 10 and the reported API values were from 51 to 200 at all stations used for the city API calculations. Within this interval, equivalent to PM 10 concentrations from 52 to 350 µg m −3 , a change of one API unit equals a change in PM 10 concentrations of 2 µg m −3 , and averaging monitoring station API values is equivalent to averaging PM 10 concentrations.
SEPA and BJEPB separately report daily city APIs using the same automated monitoring station data [50]. These reported city APIs are similar, but not always equal. Between 2003 and 2007, 1312 days (71.9%) had PM 10 API values at all reporting monitoring stations (including 98% of days with a reported PM 10 API between 96 and 105 in 2006, and 85% of days with a reported PM 10 API between 96 and 105 in 2007). During these 5 years, the official city API reported by SEPA was equal to the city API reported by BJEPB on 74.0% of days, and within 1 API value on 99.6% of days. My averaging of daily PM 10 API values from the 7 monitoring stations (8 in 2006 and 2007) gives the official SEPA city API value on 86.3% of days, and a value within 1 API unit on 99.5% of days; closer to the official city API than reported by BJEPB.
SEPA technical regulations state that the final API should be rounded to the next whole number if a decimal remains after calculation [27], however on days where there is a difference between SEPA and BJEPB values, the SEPA value is lower by 1 unit on 99.2% of days, likely due to differences in rounding. A discrepancy larger than 1 API unit has been noted between SEPA and BJEPB data when the reported API is 100 [51].
I also analyze the sensitivity of trends in the number of days exceeding the national Grade III standard with and without the monitoring station changes, and pollutants concentrations from 1998 to 2002. Annual average pollutant concentrations are analyzed during this period due to the lack of availability of daily data, and because of the change in national air quality standards.
Changes in air quality standards: in June of 2000, less stringent standards for NO 2 /NO x , TSP/PM 10 , and SO 2 were established [45,46] complicating comparisons of the number of days meeting annual standards between 1998 and 2000 and recent 2001-2007 years [41]. Specific changes include: monitoring NO x to measuring NO 2 , and the 1996 Chinese Ambient Air Quality Standards were revised [39]. The national daily NO 2 standard was raised from 80 to 120 µg m −3 , and the annual average standard was raised from 40 to 80 µg m −3 . The WHO and many other countries also measure NO 2 , and the Chinese 1996 annual average NO 2 standard was equal to the standard that would   [38]. The frequency distribution of daily PM 10 values is most often roughly log-normal [25], and analyzing data from all monitoring stations provides higher data resolution for examining potential bias.   ) to be the primary pollutant on 99% of weeks above standard. NO 2 /NO x has not been a pollutant of concern since the June 2000 change in standards, even though government reports indicate no improvement in annual average NO 2 concentrations [5,11], and studies using satellite imagery have found substantial increases [53,54]. Although the number of 'Blue Sky' days reportedly increased from 100 in 1998 to 203 in 2002, neither annual average particulate nor nitrogen dioxide (NO 2 ) concentrations improved (figure 5). Previous research noted that NO x was responsible for the largest percentage of days above the standard from 1998 to June 2000 [55] however since NO 2 began being reported in June 2000, not a single day has had NO 2 as the primary pollutant.

Sulfur dioxide
This analysis does not focus on the sensitivity of trends in sulfur dioxide concentrations to monitoring station locations, because SO 2 has only been indicated as the primary pollutant on 3% of reports above the national standard from 1998 to 2007, compared to particulate (PM 10 /TSP, 87% of reports) and nitrogen oxides (NO x /NO 2 , 10% of reports). Furthermore, from 1998 to 2007, not a single API report indicated a SO 2 level above the Grade III (250 µg m −3 ) daily standard. API

Discussion
This study examined the sensitivity of Beijing's air quality metrics by comparing air quality for 2006-2007 to previous years by correcting for the change in monitoring station locations. Three measures of air quality were used to examine trends from 2001 to 2007, including: the annual number of 'Blue Sky' days, annual average PM 10 concentrations, and the annual number of days exceeding the Grade III standard. Although the most 'Blue Sky' days is found to have occurred in 2005, the lowest annual average PM 10 concentrations and the fewest number of days exceeding the Grade III standards occurred in 2003. This illustrates that the metric used for evaluating air quality is very significant, as there can be conflicting trends based on different metrics [68].
In my analysis I calculate the impact of the 2006 monitoring station changes on the reported number of annual 'Blue Sky' days and both daily and annual PM 10 concentrations; however, due to lack of data, I was unable to  [1,4]. In 1998, the two stations monitoring transportation areas had annual NO x concentrations 100% higher than the average of the other 5 stations [1]. Given the growth in the number of vehicles, NO 2 /NO x concentrations in traffic areas have likely continued to increase [53,54]. Although street-level monitoring of NO 2 is not a suitable proxy for NO x , annual average NO 2 concentrations have been found to depend on the distance of measurement from main roads [25]. The monitoring station at Qianmen, one of the two removed traffic stations, was located adjacent to the sidewalk within 10 m of a main roadway. Resultantly, reported annual average NO 2 concentrations for Beijing in 2006 and 2007 measured without the two monitoring station locations in traffic areas are likely lower than they would have been if these stations had been included.
The 2005 automated methods for air quality monitoring which specified that monitoring stations in traffic areas not be used to measure urban air quality will likely lead to better harmonization of air quality data across China, although it complicates inter-year comparisons for Beijing [24].
In Europe, under the obligations of the European Union Framework Directive on air quality, public information on air quality is provided, separately, for roadside and background monitoring stations allowing for comparisons across Europe [58]. Within the Asian air pollution research network (AIRPET) efforts have also been made to compare air quality in major Asian cities using traffic, upwind, commercial, mixed, residential, industrial and commercial sites [59]. With vehicular emissions as a growing cause of air pollution in China, an understanding of air quality trends in these areas is especially important.
During 2006 and 2007, reporting continued for the two monitoring stations in transportations areas of Beijing, although they were no longer used to calculate the city air quality. However, on January 1, 2008 these two stations were de-listed and reporting stopped, preventing public access to air quality information for transportation areas and further complicating future analysis of trends in Beijing air quality [60].
More research needs to be done on the reported trends in air quality during the 1998-2002 periods, and the 2000 revisions to the Chinese national ambient air quality standard. Annual average pollution concentrations and the annual number of days meeting the national standard are two different measures of air quality.
Although the annual average concentrations of nitrogen dioxide and particulate did not decrease between 1998 and 2002, some of the reported increase in 'Blue Sky' days may be attributable to a decrease in the seasonal variability of pollution. As the primary source of air pollution has shifted from coal burning for heating to pollution from transportation, it is possible that annual average concentrations might not improve, while the number of days meeting the standard increases, due to less seasonal variation in vehicular emissions.
However, the impacts of the 2000 revision of the air quality standards on reported city air quality should not be understated. For example, in 1998, the annual average NO x concentration in Beijing was 151 µg m −3 -over three times the Chinese annual average NO x standard of 50 µg m −3 , and the annual average NO 2 concentration was 74 µg m −3nearly twice the 1996 Chinese national ambient air quality standard [17,61]. However, based on the 2000 revisions when the annual average standard for NO 2 was raised to 80 µg m −3 [45] the 1998 annual average NO 2 concentration was in accordance with national standards. Since the revision of standards, NO 2 concentrations in Beijing have never been above the national standard, but that does not necessarily indicate that the atmospheric concentrations of NO 2 or NO x have decreased.
Although many countries, including the United States and the United Kingdom, evaluate and publicly report the number of non-attainment days based on data from individual monitoring stations, China only widely reports averaged air quality statistics [1,3,[62][63][64][65]. In 2007, 246 'Blue Sky' days were reported for the city of Beijing using an average of air quality at eight monitoring stations in urban areas of the city, but there were only 100 days when all 27 monitoring stations in Beijing municipality reported an Air Pollution Index of 100 or less. On 265 days in 2007 air quality at least one of the monitoring stations indicated levels of air pollution above the Chinese national ambient air quality standards [4]. In 1998, 100 'Blue Sky' days were reported for the city of Beijing using an average of air quality from seven monitoring stations [5]. However, these two numbers, 100 'Blue Sky' days in 1998 and 100 days in 2007 when all monitoring stations reported air quality meeting the national standard, represent two different methods for evaluating the city air quality and highlight the high degree of sensitivity of these air quality metrics.
It has been widely reported that the number of 'Blue Sky' days in Beijing increased from 100 in 1998 to 246 in 2007, but these reported trends encompass a period during which air quality was evaluated in three different ways: (1) 1998-1999, based on the 1996 Chinese national ambient air quality standards (2)

Conclusions
Publicly reported air quality trends in Beijing during the period 1998 to 2007 are found to be highly sensitive to monitoring and reporting data. In 2007, 246 'Blue Sky' days were reported. However, if station locations had not changed, the number of 'Blue Sky' days and annual PM 10  Although nine continuous years of air quality improvement has been reported in Beijing between 1998 and 2007, my analysis finds that these improvements, as indicated by the annual number of 'Blue Sky' days, are due to irregularities in the monitoring and reporting of air quality and not to less polluted air. Reported variations in air quality that occur as a result of changes in monitoring station locations or air quality standards, should be considered as inconsistencies in the metrics and not as actual changes in air quality.